It is known, in the operation of fossil-fuel power plants and especially power plants fueled with liquid or solid fuels, to provide desulfurizing means or the like for the removal of sulfur dioxide and other noxious and toxic components from the flue gases before they are released into the atmosphere.
A typical gas-cleaning installation for this purpose will generally comprise a solid/gas separator adapted to remove particulates from the flue gases as they come from the combustion chamber, and a scrubber or washing column or the like for the wet desulfurization of the flue gases.
In the latter part of the apparatus, the flue gases are scrubbed with a scrubbing liquid containing a finely divided absorbent which can be solubilized or suspended as fine particles therein.
From the scrubbing liquid, a solid residue can be settled by sedimentation or the like which comprises the reaction product of the absorbent which is generally a calcium-containing compound. This residue thus can be rich in gypsum (CaSO.sub.4 2H.sub.2 O) and can be referred to as residual gypsum.
When reference is made to an absorbent herein, we intend to thereby designate such finely divided, particulate inorganic alkaline earth compounds, especially the calcium compounds but also magnesium compounds which can be used as an alternative to the calcium compounds but more commonly are used in conjunction with the calcium compounds. The compounds are usually in the form of the oxides, hydroxides or carbonates.
The preferred absorbent, at least the preferred virgin absorbent, as far as the present invention is concerned, is calcium carbonate. Thus when a virgin absorbent is referred to herein subsequently, unless otherwise indicated, calcium carbonate will be intended.
The reactivity and the efficiency of absorption of the absorbent to sulfur dioxide and the other toxic and noxious components can be improved by the combination of selective additives with the absorbent as will be apparent from the aforementioned applications, and the scrubbing liquid is generally water.
In most of the prior art systems, the finely divided particulate absorbent in virgin form is added to the scrubbing liquid and the recovered residual gypsum is not usable without further treatment.
The residue must be disposed of by expensive processes or treated to render it useful as a structural material. Treatments include dehydration or so-called calcination.
Mention should also be made of the fact that it is also known to effect a dry desulfurization of flue gases, i.e. to provide a process for the dry removal of sulfur dioxide and other noxious and toxic components from flue gases of fossil-fuel power plant boiler combustion chambers.
In such processes, a finely divided dry particulate absorbent or granules thereof can be added to the fuel, e.g. bituminous coal or lower rank coals such as brown coal (see German Pat. No. DE-PS 28 07 076 and the corresponding U.S. Pat. No. 4,262,610).
The fuel and the absorbent can be milled together and injected in a homogeneous form into the combustion chamber through the burner.
The solid reaction product and other particulates in the gas stream are then subjected to a dry removal therefrom, e.g. in an electrostatic precipitator, and generally speaking this solid residue is disposed of in some fashion.
The wet scrubbing process and the dry desulfurization process thus have existed side by side in the field of flue gas cleaning and have not had any type of interrelationship of significance heretofore. Both have their advantages and their disadvantages and generally one must select one of these processes or the other as the cleaning approach.
Indeed, there are a number of considerations which influence the choice of the wet scrubbing process or the dry desulfurization process.
The wet scrubbing process has the advantage of high efficiency. It operates with only a limited stoichiometric excess of the absorbent. However, it has disadvantages as well. For example, it results in a cooling of the scrubbing gas to the saturation temperature, a temperature which, in the case of the flue gases of fossil-fuel power plants is generally around 50.degree. C. This temperature is below the dew point of the flue gas. A reheating of the flue gas may be necessary for proper discharge of the gases into the atmosphere and from an environmental view point or to permit use of any other valuable component of the flue gas. Reheating, however, requires considerable energy and hence raises the cost of the process significantly whether external energy sources are used or expensive regenerative heat exchangers are installed.
The dry process has the disadvantage that the desulfurization efficiency is substantially lower and hence the stoichiometric excess must be considerably greater by many times so as to ensure effective removal of the sulfur dioxide and the other toxic and noxious components.
This is a result, at least in part, of the comparatively short residence time of the absorbent with the flue gas, even if this contact is initiated immediately upon the formation of the flue gas by incorporation of the absorbent in the fuel. The utilization of the absorbent is poor and; consequently, significant cost is encountered because of the need to use large amounts of the absorbent.
However, the amount of the absorbent which can be added to the fuel is not limited since the absorbent may overload the combustion process and such overloading thus sets an upper limit for the amount of dry absorbent which can be utilized.